1. Field of the Invention
The present invention relates to a magnetic field detecting element, and in particular to an element structure of a magnetic field detecting element having a tri-layer stack.
2. Description of the Related Art
A thin film magnetic head used for a magnetic recording device includes a playback head having a reproducing element for read and a record head having a write element for write. As the reproducing element of the thin film magnetic head, a giant magneto resistance (GMR) element is known. Conventionally, a CIP (Current In Plane)-GMR element that makes sense current flow in a direction parallel to a film surface thereof has been mainly used. In recent years, however, an element that makes sense current flow in a direction perpendicular to a film surface thereof has been developed in order to active higher density recording. A tunnel magneto resistance (TMR) element utilizing a TMR effect, and a CPP (Current Perpendicular to the Plane) element utilizing a GMR effect are known as the element of this type. In particular, the CPP element has a lower resistance compared to the TMR element, and can provide a higher output compared to the CIP element, even if the track width is narrow. Therefore the CPP element has a high potential.
The CPP element has a stack including a magnetic layer (free layer) whose magnetization direction changes corresponding to an external magnetic field, a magnetic layer (pinned layer) whose magnetization direction is fixed, and a non-magnetic intermediate layer sandwiched between the pinned layer and the free layer. Further, the stack has an antiferromagnetic layer (pinning layer) to fix the magnetization direction of the pinned layer. The pinning layer is provided adjacent to the pinned layer, and fixes the magnetization direction of the pinned layer by exchange coupling with the pinned layer. The stack may be also called “spin-valve film”. The magnetization direction of the free layer changes in accordance with an external magnetic field, and a relative angle between the magnetization direction of the free layer and the magnetization direction of the pinned layer is changed. Thus, electrical resistance of sense current that flows in a direction perpendicular to the film surface of the spin-valve film is changed. Using this property, the external magnetization is detected.
Meanwhile, in recent times, there is a desire for magnetic recording devices that have a higher recording density. In order to improve linear recording density, the layer thickness of the stack needs to be reduced. For this purpose, there has been proposed a stack having a novel layer structure different from the layer structure of the conventional spin-valve film described above. For example, U.S. Pat. No. 7,035,062, and U.S. Pat. No. 7,177,122 disclose a magnetic field detecting element having a tri-layer stack for the CPP element. The tri-layer stack disclosed in these documents includes upper and lower magnetic layers (free layers) whose magnetization directions change in accordance with an external magnetic field, and a non-magnetic intermediate layer sandwiched between the free layers. A bias magnetic layer is provided on the side of the stack opposite to an air bearing surface, and a bias magnetic field is applied in the direction perpendicular to the air bearing surface. Because the tri-layer stack does not need a pinning layer, the layer structure is simplified, and the potential to easily reduce the layer thickness of the magnetic field detecting element is provided.
To improve performance of the magnetic field detecting element, in an initial magnetization state (a state in which only the bias magnetic field is applied), the magnetization direction of the upper magnetic layer and the magnetization direction of the lower magnetic layer have to be approximately perpendicular to each other, and need to make an angle of 45° relative to the direction vertical to the air bearing surface. The more the magnetization direction of the upper magnetic layer and the magnetization direction of the lower magnetic layer become anti-parallel to each other, the more the electrical resistance value to sense current increases, and the more parallel the layers are to each other, the greater is the decrease in the electrical resistance value to sense current. That is, when the magnetization directions are perpendicular to each other in the initial magnetization state, change in an output increases in accordance with change in an external magnetic field, which can provide a large resistivity change. Further, when the magnetization direction of the upper magnetic layer and the magnetization direction of the lower magnetic layer make an angle of 45° relative to the direction vertical to the air bearing surface in the initial magnetization state, the magnetization directions take an approximately symmetric direction relative to the direction vertical to the air bearing surface even if the external magnetic field from a recording medium is applied. Accordingly, an output waveform for the external magnetic field becomes approximately symmetric relative to the output in the initial magnetization state, which improves linearity of the output in the vicinity of the initial magnetization state. Therefore, the detection performance of the magnetic field detecting element can be improved by implementing the magnetization directions as described above in the initial magnetization state.
To implement the magnetization directions in a manner described above in the initial magnetization state, the magnetization directions of the upper and lower magnetic layers are made anti-parallel to each other and parallel to the track width direction in a state not having a bias magnetic field. At this time, the bias magnetic field implements a desired magnetization direction in the initial magnetization state.
In the magnetic field detecting element disclosed in U.S. Pat. No. 7,035,062 and U.S. Pat. No. 7,177,122, the magnetization directions of the upper and lower magnetic layers are made anti-parallel to each other by exchange coupling through the non-magnetic intermediate layer when the bias magnetic field supplied by the bias magnetic layer and the external magnetic field supplied by the recording medium are not present. However, there is no mechanism that will make the magnetization directions of the upper and lower magnetic layers parallel to the track width direction. That is, the magnetization directions of the upper and lower magnetic layers may take various directions in the layer surface when a bias magnetic field is not present. The more the magnetization directions of the upper and lower magnetic layers shift from the track width direction, the lower is the detection performance of the magnetic field detection element. Further, when a plurality of magnetic field detecting elements is created, there also is a problem in which the output characteristics are different element-by-element because the magnetization directions in the initial magnetization state are different element-by-element. At this time, the number of stacks that are available as the magnetic field detecting element decreases, and productivity of the magnetic field detecting element lowers.